How to Create CSF Firewall on Linux Server

To create a CSF firewall on a Linux server, install dependencies, download CSF, run the installer, edit /etc/csf/csf.conf to add your SSH port and set TESTING = “0”, then enable CSF and LFD. Finally, open only the required TCP/UDP ports and verify with csf -s and csftest.pl for kernel/module support.

In this guide, you’ll learn how to create CSF firewall on a Linux server from scratch, configure it safely, and apply best-practice rules used on real production systems. We’ll cover installation, must-have settings, essential commands, LFD protection, and troubleshooting—written for beginners but accurate enough for seasoned admins.

What Is CSF Firewall and Why Use It?

Primary keyword focus: CSF firewall on Linux server. Secondary keywords used naturally: install CSF, ConfigServer Security & Firewall, Linux server firewall, LFD.

Prerequisites and Supported Platforms

CSF runs on most mainstream Linux distributions and VPS/dedicated servers. You need root (or sudo) SSH access, a static IP (recommended), and iptables kernel support (the iptables-nft compatibility layer is fine on newer systems). Always keep a secondary console (VNC/ILO/Out-of-Band) to avoid lockouts.

• Tested OS families: AlmaLinux, Rocky Linux, CentOS (EOL), RHEL, Ubuntu, Debian
• Disable other firewalls to prevent conflicts: firewalld or UFW (you will replace them with CSF)
• Know your SSH port (default 22 or custom)
• Have your application ports ready (HTTP/HTTPS, mail, DB, panel, etc.)

Step-by-Step: Install and Create CSF Firewall on Linux Server

1) Update System and Install Dependencies

Ensure your server is up to date and has the required Perl modules and tools.

# RHEL/AlmaLinux/Rocky
sudo dnf -y update
sudo dnf -y install perl perl-libwww-perl.noarch perl-LWP-Protocol-https tar wget curl unzip

# Ubuntu/Debian
sudo apt update && sudo apt -y upgrade
sudo apt -y install perl libwww-perl liblwp-protocol-https-perl tar wget curl unzip

2) Stop and Disable firewalld or UFW

CSF manages iptables directly. Disable native firewalls to avoid rule conflicts.

# RHEL/AlmaLinux/Rocky (firewalld)
sudo systemctl stop firewalld
sudo systemctl disable firewalld
sudo systemctl mask firewalld

# Ubuntu/Debian (UFW)
sudo ufw disable
sudo systemctl stop ufw
sudo systemctl disable ufw

3) Download and Install CSF

Fetch the official tarball, extract, and run the installer.

cd /usr/src
sudo wget https://download.configserver.com/csf.tgz
sudo tar -xzf csf.tgz
cd csf
sudo sh install.sh

Validate your environment:

sudo perl /usr/local/csf/bin/csftest.pl

If iptables modules are missing, fix kernel dependencies or ensure iptables-nft is present.

4) Prepare Safe Access: Allow SSH and Required Ports

Before taking CSF out of testing mode, ensure your SSH port and essential services are allowed in /etc/csf/csf.conf. Replace 22 with your custom SSH port if changed.

sudo nano /etc/csf/csf.conf

# Adjust these lines (examples):
TCP_IN = "22,80,443"
TCP_OUT = "80,443,53"
UDP_IN = "53"
UDP_OUT = "53,123"
IPV6 = "0"    # Set to "1" if you actively use IPv6
# For email servers add: 25,465,587,110,143,993,995
# For MySQL remote access add: 3306 (use carefully, prefer local only)

Optionally whitelist your management IP to prevent accidental lockout:

sudo csf -a YOUR.IP.ADDR.ESS "Admin workstation"

5) Turn Off Testing Mode and Enable CSF/LFD

CSF installs with TESTING = “1”. Switch it off only after SSH and required ports are set.

sudo sed -i 's/^TESTING = .*/TESTING = "0"/' /etc/csf/csf.conf
sudo systemctl enable csf lfd
sudo systemctl start csf lfd

# or explicitly:
sudo csf -e     # enable firewall
sudo csf -r     # reload rules

Confirm status and rule load:

sudo csf -s    # show status
sudo csf -l    # list current rules

Essential CSF and LFD Commands

• Reload rules after edits: csf -r
• Disable/Enable firewall quickly: csf -x / csf -e
• Allowlist IP (permanent): csf -a IP
• Temp allowlist with expiry: csf -ta IP 3600
• Deny/Unblock IP: csf -d IP / csf -dr IP
• Check logs: tail -f /var/log/lfd.log and /var/log/messages
• Uninstall (from csf folder): sh uninstall.sh

Configure LFD: Active Brute-Force Protection

LFD monitors auth logs and triggers blocks via CSF when it detects repeated failures. Key options live in /etc/csf/csf.conf; tune based on your risk tolerance and workload.

sudo nano /etc/csf/csf.conf

# Popular hardening options (examples):
LF_SSHD = "5"           # SSH failures before block
LF_SSHD_PERM = "1"      # Enable permanent blocks for SSH
CT_LIMIT = "100"        # Connection tracking limit
CT_INTERVAL = "30"      # Check interval in seconds
SYNFLOOD = "1"
SYNFLOOD_RATE = "100/s"
SYNFLOOD_BURST = "150"
PORTFLOOD = "80;tcp;20;10,443;tcp;20;10"  # rate limit HTTP/HTTPS
CC_DENY = ""            # Block countries (use sparingly)
CC_ALLOW = ""           # Allow-only list (restrictive)

After tuning, reload CSF:

sudo csf -r

Open Ports for Common Stacks (Examples)

• Web servers: 80 (HTTP), 443 (HTTPS)
• SSH: 22 or your custom port
• DNS: 53 (TCP/UDP)
• Mail: 25, 465, 587, 110, 143, 993, 995
• Databases: 3306 (MySQL/MariaDB), 5432 (PostgreSQL) — prefer local-only
• cPanel/WHM: 2082, 2083, 2086, 2087, 2095, 2096
• Plesk: 8443; DirectAdmin: 2222; Webmin: 10000

Edit TCP_IN, TCP_OUT, UDP_IN, and UDP_OUT accordingly, then sudo csf -r to apply. Only open ports you truly need.

Best-Practice Rules and Hardening Tips

• Lock in your SSH port and allowlist your admin IPs first to avoid lockouts.
• Keep TESTING = “1” until ports are confirmed, then switch to “0”.
• Use PORTFLOOD and CT_LIMIT to rate-limit abusive traffic.
• Enable SYNFLOOD for basic SYN flood mitigation.
• Don’t overuse CC_DENY/CC_ALLOW; it can create maintenance headaches.
• Monitor /var/log/lfd.log for patterns and tune thresholds gradually.
• Back up csf.conf and related files before major edits.
• In cPanel/DirectAdmin environments, use CSF’s GUI for safer changes.

Troubleshooting and Safe Recovery

• Locked out after enabling CSF? Use your provider’s console (VNC/ILO) to revert TESTING = “1” or remove conflicting rules.
• Flushed rules for diagnosis: csf -x (disable), then csf -e when fixed.
• Check module support with csftest.pl and ensure iptables-nft is installed on modern distros.
• Conflicting firewalls: ensure firewalld/UFW are fully disabled.
• Service-specific issues: verify ports with ss -tulpn and that they’re present in TCP_IN/UDP_IN.

Real-World Example: Minimal Web Server Policy

This example allows SSH, HTTP/HTTPS, and DNS resolution out, suitable for a basic web server without mail. Adjust to your setup.

# /etc/csf/csf.conf (excerpt)
TESTING = "0"
TCP_IN = "22,80,443"
TCP_OUT = "80,443,53"
UDP_IN = ""
UDP_OUT = "53,123"
LF_SSHD = "5"
CT_LIMIT = "100"
SYNFLOOD = "1"
PORTFLOOD = "80;tcp;20;10,443;tcp;20;10"

Apply and verify:

sudo csf -r
sudo csf -s
curl -I https://yourdomain.tld
sudo tail -f /var/log/lfd.log

CSF vs UFW/Firewalld (Quick Perspective)

• UFW/Firewalld: Simpler interface for iptables/nftables; fine for basic rules.
• CSF: Adds security layers (LFD, rate limiting, login tracking), richer automation, and hosting-panel integration; ideal for production hosting servers.

Need a Managed Setup?

If you prefer a hardened, production-ready CSF configuration with 24/7 monitoring, YouStable’s managed VPS and dedicated servers ship with security best practices, proactive patching, and CSF/LFD tuning tailored to your stack—so you can focus on applications, not firewall minutiae.

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